It is known to produce hydrogen cyanide according to the BMA process (hydrocyanic acid-methane-ammonia process) from an air-free gas mixture which consists of ammonia and a hydrocarbon.
As the hydrocarbon for a long time, there was preferably employed methane; more recently, however, there have also been used successfully so-called liquid gases, e.g., propane, n-butane, or isobutane, see Ullmann, Enzyklopadie der technischen Chemie, 4th edition, Vol. 9, page 359, and German patent No. 2913925 and related Voigt U.S. Pat. No. 4,289,741. (The entire disclosure of Voigt is hereby incorporated by reference and relied upon).
In both cases (Ullmann and the German patent or Voigt), the gaseous mixture is led through suspended reaction tubes made of sintered aluminum oxide, which tubes are coated internally with platinum. The tubes hang in bundles in furnace chambers and the hydrogen cyanide containing product gas mixture after passing through the tubes is quickly cooled to below 300.degree. C. in a water cooled chamber at the end of the top of the furnace, see Ullmann, loc. cit.
It is not simple to handle the reaction tubes because of the rapid temperature changes to which the tubes are exposed.
In order to be able to utilize ceramic tubes at high temperatures within industrially interesting times, there must be considered several viewpoints.
In heating to about 1250.degree. C., first there occurs a considerable increase in length. With 2000 mm long tubes of sintered alumina, this is about 15 mm. This means that the tubes, as they are arranged, can only be fastened at one end while the other end must remain freely moveable. Furthermore, above temperatures of 800.degree. to 900.degree. C., the tubes already become plastic, i.e., they remain deformed under the effect of forces acting on them. From this, it follows that the tubes are suitably suspended on one end and allowed to hang freely. The connection of the lower ends of the tubes is carried out via a soft, flexible connection which does not put up any mentionable resistance to the movement of the tubes. The gaseous mixture of ammonia and hydrocarbon enters the lower end of the tube at room temperature.
After the gases in the tube are heated to reaction temperature and have reacted on the catalyst, thermodynamic considerations require that they leave the reaction space coated with catalyst as closely as possible to the reaction temperature and be very quickly cooled in a catalyst-free condenser to a temperature below 300.degree. C. Only in this manner is there avoided a reversal of the reaction. Purely from the chemism of the process, these requirements include that the ceramic tube must be held hot effectively to its end.
The problem was solved by fastening the tube in a short piece of thermal insulation made of asbestos or similar material in a stuffing box and indirectly connecting the subsequent condenser to this stuffing box. Thus, there are simultaneously the demands to be considered to the fullest extent with respect to a marked increase in temperature and that of the ceramic with respect to a hot tube end, see F. Endter, Dechema Monographie, 1959, Vol. 33, pages 28-46.
The aforementioned considerations led to the construction of a furnace top according to FIG. 1. Suitably, the tubes 3 are placed in a free hanging position in the furnace space 2. The furnace top 1 having the under part 4 and the middle part 5 was located above them. The cooled gases were collected in the gas collection space 6. One of the tubes, number 3a, is shown in longitudinal section in order to show its fastening in the tube head holder 7.
The tubes can be sealed off against the gas collection space by a hollow shaft 8 which is shown by the number 8 for the tube 3a. The latter is absolutely important, if for example, a tube is no longer impervious in order that infiltrating fuel gases do not get into the collection space in this manner and impair the product. Furthermore, by means of the covered socket 9, there is the possibility of cleaning the tube after its withdrawal.
First, a furnace top was constructed for thirteen tubes. The tubes were cooled by using cooling coils in which there flowed cooling water.
Subsequently, efforts were made to increase the number of tubes while maintaining the old furnace construction. However, thereby one was constrained by the capacity criteria of the furnace top. Included in these were the thermodynamically favorable temperature profile, the exact mounting of the individual tubes, the good accessibility of the tubes, e.g., in sooting, as well as the simple and safe sealing of the individual tubes against the reaction space. A corresponding construction is described in German patent No. 1041476 (and related Endter U.S. Pat. No. 2,987,382, the entire disclosure of which is hereby incorporated and relied upon), which also in FIG. 1, under numbers 12 and 13, in plan view of each shows a furnace chamber terminating furnace top containing thirteen tubes in each case.
By increasing the number of tubes to twenty-six tubes per furnace chamber unit, the throughput had finally reached the geometrical limit of the tube arrangement in the furnace top, which was limiting due to capacity criteria and which, therefore, cannot be changed as desired. Otherwise, there must be enlarged the outer furnace geometry or there must be newly constructed the furnace chambers of the entire furnace and therewith there must be made allowance for considerable new investment.
Until now, it has not been possible to increase the number of tubes beyond twenty-six. In the attempts to increase the number of tubes beyond this number, there no longer is present sufficient room. Thus, the art was satisfied with the throughput obtained thereby which also appears to be the optimum for the stated temperature profile.
The temperature profile being considered is characterized by a quick increase in temperature to 1200.degree. to 1300.degree. C. after introduction of the reactants into the reaction tube and when this temperature is reached constant temperature over the entire length of the reaction tube until the gaseous mixture enters the cooling head. Here there takes place a sharp drop to below 300.degree. C. In order to protect the ceramic tube, as stated above, there is employed a corresponding insulation. The stipulation to the mentioned twenty-six tubes, however, was extremely unsatisfactory.
Therefore, it was the problem of the invention to increase the space-time-yield in the furnace available without changing the stated temperature profile inside the tube and in the cooling head and moreover that at the same time the possibilities simultaneously exist of shutting off damaged or clogged tubes from the gas collection space or to clean them easily.